ccna dummy question

ccna dummy question.NTP can obtain the correct time from internal or external time sources, including a local master clock, a master clock on the network, and a GPS or atomic clock. To prevent malicious damage to the time server, NTP uses an authentication mechanism to ensure the reliability of the time source. Time is propagated by NTP server tier. NTP servers are located in different layers (Stratum). Layer one is at the top, while layer two gets its time from layer one, layer three gets its time from layer two, and so on.12.2.2 Introduction to System LoggingWhen certain events occur on the network, the network device generates log messages based on the results resulting from the network event, and the network device has a mechanism for notifying the administrator of detailed system messages that may not be significant or may be of great importance. Network administrators can store, interpret, and display these messages in a variety of ways, and receive alerts for messages that may have the greatest impact on the network infrastructure. System log (Syslog) messages can be sent to the device's internal buffer, but can only be viewed through the device's CLI. Syslog messages can also be sent over the network to an external syslog server. The most common method of accessing syslog messages is to use the syslog protocol. Syslog uses port UDP514 to send syslog messages over the IP network to a syslog server, enabling network administrators to use logging information for network monitoring and troubleshooting. System logging is supported by most network devices, including routers, switches, application servers, firewalls, and other network devices.Each syslog message contains a severity level, which can be expressed as a number (0.7), and the syslog security levels are shown in Table 12-1. The lower the numeric level, the more severe the syslog alarm. By default, Cisco routers and switches send log messages to the console for all severity levels.Table 12.1 System Log Security LevelsSeverity Level Meaning Description0 Emergencies System unavailable (highest level)1 Alerts Requires immediate action2 Critical Critical conditions3 Errors Error conditions